How to use files in instrumented unit tests

I've got this project which processes images. The library I use to do most of the actual image processing requires met to run these tests on a Android device or emulator. I'd like to provide a few test images which it should process, the thing is that I don't know how to include these files in the androidTest APK. I could supply the images through the context/resources but I'd rather not pollute my projects resources. Any suggestions as to how to supply and use files in instrumented unit tests?